NEW YORK -- SBC Communications Inc. agreed to buy the wireless division of Comcast Corp. for $400 million in cash plus the assumption of $1.3 billion in debt, reflecting the quickened pace of consolidation in the cellular business.

The acquisition, expected to close in the third quarter, underscores a race by the regional Bell operating companies to provide national coverage at affordable prices. AT&T Corp. and Sprint Corp. are taking significant market share from the Baby Bells by offering flat-rate, nationwide...